Abstract
The effects of cross correlation between dipolar and chemical-shift anisotropy relaxation interactions on the measurement of heteroatom T 1 and T 2 relaxation times in proteins is considered. It is shown that such effects can produce errors of approximately 25% in the measurement of 15N transverse relaxation times at a field strength of 11.8 T. Cross correlation has a less significant effect on the measurement of 15N spin-lattice relaxation rates and for proteins the errors in T 1 decrease as a function of increasing molecular weight. Nevertheless, for T 1 measurements at 11.8 T errors of approximately 15 and 5% are calculated for proteins with correlation times, τ c, of 5 and 9 ns, respectively. Pulse sequences which eliminate dipolar and chemical-shift anisotropy cross-correlation effects are described. These sequences are used to make more accurate measurements of 15N T 1 and T 2 values of staphylococcal nuclease and to determine errors in these parameters that result when cross correlations are present.
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